Abstract The senescence of tissue stem cells, including that of hematopoietic stem cells (HSC), is a major underlying mechanism for aging of an organism. Despite extensive analysis, the underlying cause for senescence of HSC remains largely unresolved. Although it is generally accepted that both cell intrinsic and environmental factors are involved in senescence of tissue stem cells, how the environmental factor in the aging organisms translates into senescence of their HSC is unclear. The mTOR pathway has emerged as a major sensor for environmental and nutrient conditions of many cell types. In the past several years, my laboratory has been investigating the role of mTOR in the function of HSC. Our preliminary data demonstrated that the mTOR pathway is over-activated in the aging HSC and that such activation is both necessary and sufficient to cause functional defects of HSC, as well as HSC senescence. The underlying cause of mTOR activation remained unknown. Here we plan to take biochemical, genetic and immunological approaches to identify the cause of mTOR activation in aging HSC. As part of our effect to demonstrate essential role for mTOR activation in HSC aging, we showed that a six-week treatment with rapamycin can rejuvenate the aging HSC. This result raised an intriguing potential to translate our fundamental discovery to clinical care of the increasingly older population. Our proposed studies are detailed in two specific aims. Specific Aim I. To identify the root cause of enhanced inflammatory cytokine production and HSC senescence in the old mice. Specific Aim II. To identify a potential therapeutic window for transient use of low dose rapamycin to rejuvenate aging HSC. This application is based on our several lines of exciting and innovative studies in TSC-mTOR pathway in HSC function and in HSC senescence, as well as a novel pathway in regulating immune response to cell injury published in J. Exp. Med., Science Signaling and Science in past two years. Our proposed studies will not only elucidate a fundamental mechanism of HSC aging, but also provide novel approaches for therapeutic rejuvenation of HSC. The completion of the proposed study will have both immediate and long term impact for clinical application.